Field of the Invention
The present invention relates to a semiconductor DBR, a semiconductor light-emitting device, a solid-state laser, a photoacoustic apparatus, an image-forming apparatus, and a method for manufacturing the semiconductor DBR.
Description of the Related Art
A vertical-cavity-type surface emitting laser (vertical cavity surface emitting laser, VCSEL) is a laser including a cavity formed on a substrate in the vertical direction of the laser, the cavity being constituted by two reflectors and an active layer interposed between the two reflectors. As a reflector, a Bragg reflector (distributed Bragg reflector, DBR) including two layers having different refractive indices, the two layers being alternately stacked on top of one another a plurality of times and each layer having an optical thickness corresponding to ¼ wavelength, is used. The layers constituting a DBR are composed of a dielectric substance or a semiconductor and, in many cases, a semiconductor is employed from the viewpoint of formation of a device.
The thickness of an active layer of a VCSEL is very small, which makes it difficult to achieve a large gain. Therefore, the reflectance of reflectors constituting a cavity is desirably increased to 99% or more. In order to increase the reflectance of DBRs that serve as reflectors, the number of the pairs of the two layers having different refractive indices and a difference in the refractive indices are desirably increased.
In the case where a DBR constituted by semiconductors is formed by epitaxial growth, a difference in lattice constant between a substrate and a semiconductor to be epitaxially grown on the substrate is desirably reduced. A large difference in lattice constant causes lattice strain, which causes an in-plane stress in the epitaxial film. This may lead to degradation of the surface smoothness of the semiconductor layer and occurrence of cracking in the semiconductor layer.
For example, a tensile stress occurs in an AlGaN layer formed on a GaN substrate, and a compressive stress occurs in an InGaN layer formed on a GaN substrate. In Japanese Patent Laid-Open No. 2000-349393, a semiconductor DBR is formed on a GaN substrate by alternately growing an AlGaN layer and an InGaN layer on top of one another at 800° C. Thus, in Japanese Patent Laid-Open No. 2000-349393, occurrence of cracking is suppressed by alternately stacking an AlGaN layer and an InGaN layer on top of one another, thereby compensating for lattice strain.